The development of an in vitro system for investigating the hormonal regulation of spermatogenesis is proposed. Endocrinological similarities in the control of spermatogenesis in mammals and fish permit analogies to be drawn regarding hormonal regulation of the processes. It is hypothesized that the anatomical and physiological characteristics of Fundulus heteroclitus (the common killifish) testes will enable germinal cells in testicular fragments to be stimulated to undergo meiosis and/or spermiogenesis in vitro, thus providing a simple model for mammalian spermatogenesis. The testicular recrudescence of Fundulus, reproductively synchronized by manipulation of temperature and photoperiod in the laboratory, will be examined. Gonadotropin levels in the pituitaries of fish with regressed and recrudesced testes will be determined. Qualitative and quantitative changes in testicular androgen metabolism, in vivo, will be correlated with morphometric analyses of the percentages of different spermatogenic stages of germinal cysts appearing in testes at discrete time points during the process of recrudescence. The optimum temperature for culturing Fundulus testes will be determined by measuring the length of time required for labeled thymidine to appear in spermatozoa at different temperatures in vivo. In vitro, testicular fragments containing germinal cysts of differing spermatogenic stages from reproductively synchronized animals will then be cultured to see if: 1) spermatid cysts will poduce spermatozoa, 2) spermatocyte cysts will produce spermatids, and 3) spermatogonial cysts will produce spermatocytes. The attainment of these objectives will determine the basic endocrinological changes associated with the anatomical alterations occuring during spermatogenesis in Fundulus testes, establish which stages of spermatogenesis are androgen dependent, and create a defined system for examining the hormonal regulation of the interactions of germinal cells with other testicular cells.